1. Field of the Invention
The invention relates to a method of forming a high temperature oxidation resistant coating on a nickel-chromium alloy body. The method includes incorporating into a nickel-chromium alloy a sufficient amount of cobalt to effect a change in coating morphology when the alloy is aluminized, and thereafter subjecting the body to a coating of aluminum.
2. Description of the Prior Art
While aluminide coatings of superalloys which generally contain iron, nickel, cobalt or chromium, singularly or in combination as the basis for their composition, as well as one or more additions of elements, such as molybdenum, tungsten, rubidium, titanium, tantalum and/or aluminum for the express purpose of solid solution strengthening are known, the general mechanical integrity characteristics of aluminide coatings of superalloys cannot be projected to non-super-strength alloys, e.g. nickel-chromium alloys, since non-super-strength alloys being free of solid solution strengthening elements have different morphologies, e.g. microstructures, than superalloy morphologies.
Heretofore, it has been reported by L. A. Monson et al. Technical Report AFML-TR-66-47, Part I (March 1966), that aluminide coatings on certain nickel-chromium alloys (thoria dispersed nickel-chromium alloys) were extremely porous. Porosity has been attributed to the unequal diffusivities of nickel and aluminum with nickel leaving the substrate and diffusing out faster than aluminum diffuses in from the surface. This unequal diffusion flux apparently results in vacancies which appear to coalesce, quite likely on the few larger thoria particles forming voids.
In general, spalling of the aluminide coatings on nickel-chromium alloys is associated with porosity at the coating substrate interface. Heretofore, the exact cause of delamination between the outer and inner region of the aluminide coatings on nickel-chromium alloys has not been fully understood. In general, however, it was believed that delamination occurred at or near a thin continuous region of a high .alpha.-chromium phase that developed during the aluminizing of the nickel-chromium alloys. Further, it was believed that the high .alpha.-chromium phase tended to be very brittle which lead to delamination of the coating caused by fractures along or near the high chromium phase due to stresses resulting from a thermal expansion mismatch between the coating layer and the substrate. Still another possible cause of delamination was believed to be the formation of Kirkendall voids at or near the region of the high chromium phase. Presumably the high chromium phase acted as a "diffusion barrier" to the inwardly diffusing aluminum atoms, while acting as a "diffusion medium" for the outwardly diffusing nickel atoms. A layer of voids at or near the high chromium phase was believed to lead to a loss of contact between the coating and the substrate.